The protein alpha-synuclein (aS) is implicated in the etiology of both familial and sporadic Parkinson's disease (PD). The interplay between the normal function of aS and its pathological aggregation is poorly understood, but membrane-bound forms of aS are thought to mediate its physiological function, while aggregated forms are thought to mediate the toxicity of the protein. Structurally, aS is highly malleable, adopting a highly disordered conformational ensemble when free in solution, highly helical structures when bound to phospholipids membranes and b-sheet rich conformations when aggregated into amyloid fibrils. Preventing the aggregation of aS into amyloid fibrils or potentially toxic oligomeric species is a promising strategy for the treatment of PD. The overarching goal of this research is to achieve a detailed understanding of how synuclein structural properties and transitions modulate synuclein function and toxicity and to identify specific conformational states of aS that could facilitate the design of synuclein-interacting reagents with potential therapeutic value. The current proposal is aimed at filling a newly emerged gaps in our understanding of aS structure that were created by a) the discovery of a new PD-linked aS mutation, E46K; b) the discovery that membrane-bound aS can adopt two different topologies, an extended helix and a broken helix, and the formulation of a hypothesis regarding how these two conformations may influence synuclein function; c) the discovery of new aS interaction partners thought to modulate synuclein function. To fill these gaps and to address emerging hypotheses we have developed the following specific aims: 1. To determine the effects of the most recently discovered PD-linked mutation, E46K, on structure in the free and membrane-bound forms of aS. 2. To elucidate at high resolution the extended-helix structure of membrane-bound aS. 3. To test the hypothesis that aS can mediate interactions between topologically distinct membranes of different compositions using its previously elucidated broken-helix structure. 4. To determine the effects of AARP16/19 binding on the structure of membrane-associated aS. These aims are motivated by the opportunity to clarify how aS sequence variations influence the structure and aggregation of the protein, and by our belief that monomeric membrane-bound conformations of aS, which are more highly ordered, may be better suited to form specific interactions with potential therapeutics. This work will advance our understanding of aS structure, function, and aggregation and will provide a structural basis for the future design and identification of reagents that can stabilize monomeric aS and prevent its oligomerization and aggregation. Furthermore, the results obtained may have general implications for strategies to address protein aggregation in other age-related motor disorders and dementias.